1. Field of the Invention
Embodiments of the invention described herein pertain to the field of marine transfer of liquefied natural gas (“LNG”) between vessels. More particularly, but not by way of limitation, one or more embodiments of the invention enable an apparatus, system and method for a retractable LNG cargo transfer bow manifold for tandem marine cargo transfers.
2. Description of the Related Art
Natural gas is often carried in liquefied form onboard special cryogenic tanker ships from the location of its origin to the location of consumption. In this way, natural gas may be transported to areas with a higher demand for natural gas. Since liquefied natural gas (LNG) occupies only about 1/600th of the volume that the same amount of natural gas does in its gaseous state, liquefying the natural gas for transport facilitates the transportation process and improves the economics of the system. LNG is produced in onshore or offshore liquefaction plants by cooling natural gas below its boiling point (−259° F. at ambient pressures). The LNG may be stored in cryogenic containers either at or slightly above atmospheric pressure. Typically the LNG will be regasified prior to distribution to end users.
In some instances, a mobile vessel is loaded with LNG cargoes at the natural gas supply source and travels across the ocean to another location for offloading and distribution. Increasingly, an LNG carrier (LNGC) or vessel with regasification facilities is loaded with LNG cargo at a location between the port of origin and the port of consumption using ship-to-ship (STS) transfer of LNG. In one example, a conventional LNGC collects the LNG from the liquefaction plant at the natural gas supply source or other LNG loading location and is used for the long haul or a portion of the transportation route. The conventional LNGC delivers the cargos from the supply source to the STS transfer location. In this example, a regasification vessel receives the cargo from the LNGC and may be used in shuttle service between the STS transfer location and the offloading port. In yet another example, a conventional LNGC berths alongside a floating platform for the regasification of LNG onboard the floating platform, and the floating platform is attached to a riser or jetty. In such scenarios, STS transfer is used to load LNG onto the floating platform.
In order to implement the STS transfer of LNG, it is often advantageous to use a tandem loading configuration, in which the stern of one of the vessels faces the bow of the other vessel, as illustrated in FIG. 1. The actual STS transfer of the LNG may be either from or toward the vessel astern of the other. In FIG. 1, loading device 100 is shown on conventional loading deck 110 on conventional receiving vessel 130. The raised position of loading device 100 is permanent and allows conventional LNG cargo transfer hose(s) 115 or articulated pipeline (arms) to remain in a stable catenary/apex configuration, and also provides for conventional disconnect coupler 120. Conventional disconnect coupler(s) 120 allows tor immediate separation of supply vessel 125 and receiving vessel 130 in the event of an emergency during an STS transfer operation. In such circumstances, the flow of LNG ceases, and the conventional transfer hoses 115 be disconnected from the supply 125 and/or receiving vessel 130 at the conventional disconnect coupling 120.
Vessels, including LNGCs and offshore floating platforms, must be capable of withstanding severe weather conditions, such as storms, high wind and hurricanes. During extreme weather conditions, it may not be possible or desirable to move a floating LNG production vessel to a protected location due to moorings, gas risers, and/or pipeline and well controls, and so the vessel may experience heavy loads from large waves beating down on the deck of the vessel. In the case of the transporting LNGC the effects of ocean, wind and wave systems during significant storm events during a passage, may physically damage a fixed bow loading or fixed bow unloading unit to the extent it is no longer safe to use at the next loading or disport facility. Strong or continuous wave forces on decks can cause damage to the vessel and equipment on board, and the deck may be flooded with green water (a compact mass of water flowing across the deck of the vessel).
To combat damage during extreme weather conditions, oil bow loading units are typically robust manifold systems capable of weathering stormy seas. However, unlike oil, LNG is cryogenic. For LNG transfer tandem configurations, the transfer systems are not as robust in size and mass and are therefore subject to damage not typically sustained by oil units. Unlike oil loading/offloading devices, these LNG handling systems may include particularly fragile process instrumentation, emergency shutdown systems and/or quick connect/disconnect couplers which are a required design component in order to meet safely standards (e.g., ISO DTR 17177, ISGOTT, SIGTTO STS Transfer Guide). Thus, fixing the LNG loading/offloading device on the bow of the LNGC during travel risks significant damage to the loading and unloading units.
Conventional LNG bow loading and conventional LNG bow unloading units currently used onboard LNG vessels are not well suited to withstand extreme weather conditions. Therefore, there is a need for a retractable LNG cargo transfer bow manifold for tandem marine cargo transfers.